Apparatus for dielectric heating



y 26, 1953 M. P. VORE ETAL 2,640,141

APPARATUS FOR DIELECTRIC HEATING Filed 001;. 12, 1948 2 Sheets-Sheet lFig.l.

INVENTORS WITNESSES Milton P.Vore and rgbm E.Kirby.' fi :2 a xzagmwATTORNEY May 26, 1953 M. P. VORE ETAL APPARATUS FOR DIELECTRIC HEATINGFiled Oct. 12, 1948 2 SheetsSheet 2 Fig.2.

INVENTORS WITNESSES:

11 1 WWW Q 0' mm W .1 7 m P 2n n H A m x H Y. MM

Fig. 3 is a plan view of the preferred form of our noveldielectric-heating electrode shown in Figs. 1 and 2;

Figs. 4 and 5 are detailed sectional views on the lines IV-IV and V-V ofFig. 3; and

Figs. 6 and 7 are perspective views of two forms of bars used inbuilding the preferred form of our dielectric-heating electrode, towhich, however, we are not limited.

The embodiment shown in Fig. 1 comprises a pair of opposed generallyhorizontal electrodemeans which are indicated in their entireties by thereference numerals 2 and 4, respectively. These electrode-means arevertically spaced to provide a space therebetween which serves as awork-receiving passage. The electrode-means 4 comprises a horizontalmetal plate 6 having turned ends 8. An endless conveyor 10 of insulatingmaterial has an upper run which rides on the plate 6. The conveyor iswrapped around drums 12, one of which may be driven by any suitablemeans to move the upper conveyor-run in an assumed direction shown bythe arrow in Fig. 1, at the right. The conveyor carries suitabledielectric material l4 through the work-receiving passage between theelectrode-means 2 and 4. The plate 6 and the conveyor I0 form awork-supporting means by means of which the material [4 is supported andcarried through the apparatus in a linear travel-path. The material isof less height than the extent of the space between the electrode-means2 and 4, so that an air-gap I5 is present between the top of thematerial l4 and the bottom of the electrodemeans 2.

The embodiment shown in Fig. 2 comprises generally horizontal opposedelectrode-mean 2 and which are vertically spaced to provide awork-receiving passage therebetween. The electrode-means 20 comprises awork-supporting means in the form of metal rollers 22 which are rotatedby any suitable means for conveying material 24 thereon through thework-passage in the assumed direction indicated by the arrow at theright of Fig. 2. The material 24 may be the and 32 may be made either ofmetal or of insulation, depending on whether the electrodemeans 2 is tobe grounded or insulated from ground.

Secured to the lower ends of the support-arms and 32 are horizontal stubshafts 34 journalled in suitable fingers 36 which carry a metal frame38. The frame 38 is part of the electrode-means 2. Accordingly, eachpoint of the electrodemeans 2 can swing back or forth in a verticalplane paralleling the direction shown by the arrows in Figs. 1 and 2, inwhich the work travels. The parallelogram-like support of theelectrodemeans 2 allows the electrode means to remain in a horizontalplane as it swings. Obviously, cables, wires, etc. will allow for aneven greater freedom of swinging and other movement.

The electrode-means 2 also comprises a plurality of support-membersdependently carried by the frame 38 and a plurality of electrodebarspivotally carried by the lower ends of the depending support-members 40and subsequently described in greater detail.

The main purpose for showing the two em bodiments of Figs. 1 and 2 is toillustrate the adaptability of the upper electrode-means 2, whichconstitutes our novel dielectric-heating electrode, with different formsof lower electrode-means of which the electrode-means 4 and 20 serve asillustrations. Thus, for example, a conveyor, such as I0, can be made ofmetallic material and itselfprovide the lower electrode. The showing ofthe two modifications is also to illus trate the adaptability of theelectrode-means 2 for use in different positions with respect to thetravel-path of the material to be heat treated, that is, with respect todirection in which the material is moved by any suitable conveyor means.With such travel-path as a fixed reference, it is apparent that theelectrode-means 2 is shown in Fig. l disposed at a right angle withrespect to its showing in Fig. 2.

As is known to the art, in a system set up as shown, the electrode-means2 is electrically in sulated from the electrode-means 4 of Fig. 1 and 20of Fig. 2. Such relatively insulated relation between the respectivepairs of electrode-means can be obtained in any suitable way. As anexample, Fig. 1 shows the electrode-means 2 grounded. Consequently, theplate 6 of the electrode-means 4 of Fig. 1 should be insulated fromground, and the support-arms 30 and 32 can be of metal. As a secondexample, Fig. 2 shows the electrode-means 2 insulated from ground.Consequently, the electrode-means 20 can be separately grounded, asshown at 42, and the support-arms 30 and 32 can be insulated, or made ofinsulating material, or any other suitable expedient used. Obviouslyother combinations are possible, as for example the electrode-means 2 ofFig. 1 can be insulated from ground and the plate 6 of theelectrode-means 4 grounded. Additionally, instead of a groundedcircuit-path, a conductor can be substituted.

Assuming, as shown in Fig. 1, that the plate 6 is insulated, ahigh-frequency electric field is established between the plate and theelectrodemeans 2 by electrically connecting power delivering conductors44 and 46 to the plate 6 and electrode-means 2, respectively. Theconductors 44 and 46 receive power, either directly or indirectly, froma high-frequency oscillation-generator 4B. The conductor 46 is shown,for convenience, as grounded at 49. For completing the electric circuit,the electrode-means 2 can be grounded either through the member whichsupport it or through a separate grounding conductor connected through aterminal connector associated with the electrode-means in any suitablemanner.

The physical construction of the electrodemeans 2 is more particularlyshown in Figs. 3 through '7. The frame 38 of the electrode-meanscomprises support-bars in the form of two spaced, lengthwise channels 50and 52 interconnected by a pair of outer channels 54 and 56 and an innerchannel 58. The channels 54, 56 and 58 are provided with slots 60, 62and 64, respectively, the slots extending lengthwise in the base 66 ofthe channels centrally between the leg 68 of the channels. Each slotreceives a plurality of the support-members 40 which carry a pluralityof electrode-bars. The electrode-bars :58 'cumprisenarrovestripswor:shapeszsuclnasmhown 6 andi'l. The electrode-beretFigsfioomprises excomzpzutatively @short strip ml halving anoenter'zhole l2and a pair of end holesi flk and aflfi whiclrai'e roversize- :orelongated. Q Y-The: othemelectnodeebar, @lshownl ini Fig; 7; comprises azstiti i vlll sw-hich dis approximately twice as long as-theistrip lll..fThe v strip? ill-sis vprovided::with I a oen-ter hole: 80; andover-size zor. elongated holes B2 anti- 1M atthe emis 1 or the :strip.The strip 'l8,-.,has"-further h0les186 1 ohm 88; respectively halt -wayh-etween the' -v holes 8U' -flIld=-82, end-theholes 80 end ':84.Preferably, the v-electrode-loeus 'have rounded edges to: avoid sharppohits, and theinoenter end end: holessare -counters-unl: on=thei1=under sides.

As 'shown' in' Fig'; 3;"- the electrode-bars or strips arranged to forma lazy-tong structure. To 'this end; i the strips e.re--arrenged' in-"two :levels emeteries-cross ea'ch otherp' The-*strips*inthei lowerlevel comprise the long parallel *strips 18 end i diagonally oppositesh-ort strips *70. The strips in "the upper level cm'npriselongyparallel strips 18 and diagonally opposite strips 'lfi'. The

bottom surfaces of thestripsin eech levelprefer- 1 can be'movecltooompressthe spring-l8?! so es-to ahly lie I insingle plane; withiirmanuiaeturing end assembly tolerances which in most cases allowconsiderable leeway.

\ In the arrangement shown; each lower strip +18; rexcept 'the oneettheupper end of. the leizytong arrangement,"crosses three upper-strips *'-'-r-e' at intermediate points-along the length 'of the stripI8, and meets the ends of tWOstripsTB end pointsof the seinestrip'lt.Likewise, an

ripper strip 18 in effecterossesand meetshfive lower strips '78.Accordingly, exceptior'the ,out- --er1nost strips; strip it of the lowerlevel of the lazy tong structure hasfive'junotioir points with th strips18'"of'theupperlevel, and conversely afsimilar strip "1 8 "10f the upperlevel of the lazytong has five junction points with the lower strips'78. Three -"ofthe junction points forsuch strips are etthe channels54-, -56 andSB, and'two are centrallybetweenthefchannels.

Eacnortheshort end-stripsJfi andfl) crosses ,bzir 7'18 01" 18' at a,point which is'centrallyba "twcenthe innerchannel 58 andganouter'channel 54 or 56. Each oftheenwstrips 7,0 and I'll) has a"junction point at'iitsend whichis at -the channelfifi; and. a similarpoint etits end which iset a'channelfMor'SB.

The criss-crossed" arrangement of. the electrode-strips 10, 1,6", "68andglfilis carried from the channels "54,156, anri'lia by a; pluralityof pivot means comprising ;thes upportmembers 40. ,A

support-member "M1 is provided :at each junction point of the bar i8 anditi'that lies at afchennel 'd, 156 or .58. Similar support-members..,areused ateach junction point 'of the bars in and i0 and at the junctionpoints of these bars H! and nrand a bar 18 or-"IB'. In'the preferredembodiment, the support-1nembers-are substantiellye'lilze and areconstructed as shown in Fig. 4.

Referring in' particular tol'Fig. A, a support member"Ml-"comprisese-flathea d brass screw 90 having-an end that passes through the slot 64"inthe chemnel tt. Thelower headend of the screw passes through alignedholes in the strips 711.8 and 758, withithe head of the screw-seatingvinn-the; countersunk hole in the lower strip 7-8. ,r-srbrnss washer "isZlies between the crossing elecatrodeebersr 1 A stop nut 29.4;pressesa-isprin rwashrer- 96o qagainstltheqtop. of: thenpperistrip J8? .and

" the -screw '68 iree-lwpasses, pair of: stub aShEtftS 'l' 'si Z emiMex-tending in opposite directions outdim 1 call contactiwith? the brasswe shenhut freeenough xso that they oen twist -erounchthescrew 90.

whe screwullll carries a thrustunut 98- sli-ght1y shove. the stopvnut 1a compression springl00 above the thrust nut;- em -lower roller meansilfll' *ebo've the spring lfifl and "below the chennel 58, =ensippe1=ro1ler means Mil: shove I r the I channel, and locking nuts i136 e-nditt.

Each of the roller meaJ-ns IiJE a-nd "i 34 comprises aplate support llil having a hole'through which and flScarried rcspectivelywn the "stubshafts l-i'2-and-"i"hl; and suitable retaining '3118811'195501 tsnce ofthe electrootehars it, "lflQ ltenda'm ebovet-he-lower-electrorie meenstor "it; as the "case-may be;-oan be controlled. -Moreover;:., the

erent'j'unction'pointsof the bars can be inuelly'e justed so as to makethe; surface of "the electrode strips as level as convenient-0r desired.After such adjustment;the-thrust nut-'98 determine the force: with whichthe roller means encl i'fifi presseeainst the bottom and topgof thechannel To good el ctrical conduction ,between erchennel and the roller,means, smooth narrow "brezss strips eresecurecl to eachchannel 5 4,

tile-oats along-eschside of the slo'tsj therein, coextensivetherewith.'il'le'strips l'lg may be "on the upper or-onthelower sidesjofrthechamriels or: on is 0th sides. In, the embodimentsshown, they ere o'hlyyon.the lower sides. The rollers H6 and l 18 "of the :lower roller means 192ride through the rollers to, the screws'ffiil .land the 1ps'l0",19',l8and'l'il. :At each of the junction pointsof the, electrode- :st 'ps,between'ithe'channel members, la wpivot "r241 provided for holdingiithestripstoge'ther, and to. provide go'odgelectrieal,contact therebetween..A preferred ,form of construction for such pivot means is shown moreparticularly inFigj. '5; It comprises awshort vbrassscrewJZ-E whichseats in the ocountersunkoseatoof" a .lower electrode stripl .enci.passes lthrough the crossin strips;. being, held. by. estop nut 12%..pressing against a spring washer i3fl. {I'o allowforgreater freedom, ofmovement .between.,,the crossing strips and for good electricaLcontect,therebetween, a brass, washeri (3210911 beprovided, gthere== between.

Fig. 3 shows the. l lazy-tong 4, arrangement ,of veleetrod.e.,b2n'sslightly-i extended. Thev upper strips-T181 are allparallel and equally;spacedmavin dtheireends at theslotsin the outer. cha-nnels Mandffrfi.These stripsareoblique to thesecham .nels but. deviate only slightlyfrom 2 perpendicu- 1a]: "thereto. The lower stri s r 181 obliquely icross the. ,upper strips 18? and elso,,.have their end points attheislotsiin the outer-channels Blend 56. ".The inneriuchannel 5,8 serVesas-a, further supporting .pointjfor each. strip. 1-" The intermediateKpivot p0ints"I2G,he1p.t0 prevent sag inthe strips at the (junctionpoints :between channels. l The-crossing strips provide .a, perforatedelectrode-means, having, splurality oftdiamondmshaped openings. {Theopenings tarerarrensed in rows with the Qnenings ,m a rowbeins staggeredwithzresp ct athepe ringstin. the. edj,acent-.;r.0w ,.=;,theastesser nrbein vtimes, the two-strips :18 land118'-t,-intoagood;.electrig ..,suchthat no strai ht-linemanibedrasvnzacrqssthe electrode-means withoutcutting across an opening. It is also to be noted that each openingprovided by the criss-crossed strips has nonuniform lengths at differentparts thereof, measured in parallel directions.

Preferably, a large number of strips are provided in order to producethe electrical effect of an electrode-means consisting of a solidmetallic sheet. It is known that a perforated metallic sheet can providethe effect of a solid piece of metal for bounding an electric field ifthe openings in the structure are not too large. In a typical lazy-tongstructure, such effect was obtained by using long aluminum strips ofclose to 60 inches in length, and short aluminum strips slightly morethan half that long. The number of strips used correspond to the numbershown in Fig. 3, in a frame having dimensions that were approximately 56inches by 46 inches, the shorter dimensions being that in which thelazy-tong structure could be expanded and contracted. With thearrangement of the electrode-strips in the manner shown, with their flatfaces facing the opposed electrode-means 4 or 20 and their narrow edgesupstanding, the strips as a whole can be said to lie on the same levelwhen they are spaced far enough from the opposed electrode-means 4 or 20and they are sufficiently thin. In the specific embodiment described,the strips were 3% inch thick, and the work-receiving passage 1 to 3inches high.

Obviously, by loosening the thrust nuts 98, the lazy-tong structure canreadily be spread out or collapsed for increasing or decreasing thecrisscrossed area encompassed by the structure. The nuts can then beretightened. Loosening the nuts I05 and I08 also permits the lazy-tongstructure to be extended or contracted, but may require more caution inorder not to change the vertical spacing between the electrode-means.

The holes 74, I6, 80, 82 and 84 in the electrodebars or strips aresufficiently oversize or elongated to allow the lazy-tongelectrode-means 2 to be adjusted so as to occupy any desired portion ofthe area of the frame 38, from a maximum determined by the length of theslots 60, 62 and 64 to a minimum determined by the width of the strips.By adjusting the angle which the strips make with each other, andcorresponding with the angle which each strip makes with the channelmembers of the frame, the effective area for the electrostatic field ofthe electrode-means 2 is correspondingly changed.

If the electrode-means 2 is arranged as shown in Fig. 1, the overalllength of the electrostatic heating field can be changed in thedirection of movement of the material I4. Such an arrangement is morelikely to be used where the width of the material being heat treatedremains fairly constant. If the electrode-means 2 is disposed as shownin Fig. 2, the overall width of the electrostatic heating field can bechanged in a direction transverse to that in which the material 24moves. Such an arrangement is more likely to be used where the width ofthe material being dielectrically heated will vary from time to time andit is desirable to adjust the electrode-means to a corresponding width.In this latter arrangement, should an arc strike from the material toone of the electrode-strips, it will ultimately reach an opening formedin the criss-cross arrangement of the strips. As the point of origin ofthe arc moves into the opening, the arc elongates and becomesextinguished. In the meantime, the spot of glue or other material whichcaused the arc to strike has .a chance to become burned out.- For such76 arc-extinguishing operation, it is desirable to provide a minimumspacing between electrode-strips on the same level,'the spacing beingsuch that an opening will always be encountered by an are moving withthe material.

Oblique strips in a single layer only also provide an open space forelongating any are occurring in the air-gap I6.

If a more direct contact for an electrical circuit to theelectrode-means 2 is desired, one or more of the electrodesupporting-members 40 can be provided with a terminal connector I34, asshown in Fig. 4. The connector is held in place on a screw by a holdingnut I36. The terminal connector I34 receives a conductor I38 which maybe insulated or grounded so as to correspond to the conductor 46 ofFig. 1. In order to carry the electricity readilyfrom the connector tothe various electrode-strips, the parts of the electrode supportingmembers 40 that may be called up to carry current are preferably made ofbrass, and for economy the channels of the frame 38, and the variouselectrode strips Ill, 10', 18 and 18 can be of aluminum.

There is a distinct advantage in providing a plurality ofsupport-members 40 for carrying the criss-crossed strips. The relativelyelongated screw 90 of each support-member allows for limited verticaladjustment of the levels of the strips 10, 10', 18 and 18, throughraising or lowering the holding nuts I06 and I08. The nut 98 can beadjusted for a suitable compression of the spring I00.

A swinging support for the lazy-tong, such as provided for Fig. 2, andobviously also applicable to Fig. 1, also has a distinct advantage incase any material being dielectrically heated should buckle. Theelectrode-means 2 can swing when a piece of buckled board should hit it,a may sometimes occur in edge gluing where a plurality of elongatednarrow veneers or boards are placed lengthwise on the conveyor meanswith their upright edges to be glued together. A safety switch such asI40 can stop the conveyor motor through a motor controlling relay I42,when the electrodemeans swings. The buckled boards can then be removed,or the electrode-means swing out of the way to permit the boards topass.

While we have described our invention in fairly particular detail, it isobvious that these details are subject to wide variations and that manymodifications can be made thereof.

We claim as our invention:

1. Dielectric heating apparatus, comprising, in combination, a pair ofparallel, generally horizontal, relatively insulated electrode-meansvertically spaced to form a work-receiving passage, the upper one ofsaid electrode-means comprising a plurality of elongated obliquelycrisscrossed strips having faces lying substantially in a plane, thestrips being carried with said faces adapted to bound a high-frequencyelectric field, fastening means for securing said strips together, andconductor means for applying high-frequency power between said pair ofelectrode-means.

2. Dielectric heating apparatus adapted for the continuous gluing ofveneers of wood, comprising, in combination, a lower work-supportingconveyor means compri ing a lower electrode-means, said conveyor meansbeing adapted to move wood in a predetermined linear travel-paththerealong, a plurality of metallic strips having narrow edges and flatfaces, supporting means supporting said strips along the top of saidtravel-path with the strip-edges upstanding and the strip-faces lying ina-substantially flat surface paralleling "said said supporting means-oomprising adjustable *mech-anisnr for changing thespacing between saidstrips" lor raisinga-nd lowering "them,

and means for relativelyinsulating said lower "electrode-means and saidstrips.

3. Dielectric heating apparatus adapted for the continuous gluing ofveneers 'of-wood comprising, in combination, a lower work-supportingconveyormeans comprising alower electrode-means "said conveyor meansbeing adapted to move wood in a travel-path therealong; aflirame abovesaid conveyor means, .said iramecornprising. a plurality of spacedelongated support-bars a plurality of metallic electrode-strips havingflat surfaces, and pivot means associated with said supportbars and saidelectrode-strips, and carrying said electrode-strips from saidsupport-bars in a lazy tong arrangement, with the flat surfaces of thestrips facing said conveyor means.

4. An invention including that of claim 3 but further characterized bysupporting means swingingly carrying said frame above said lowerelectrade-means.

5. Dielectric heating apparatus adapted for the continuous gluing ofveneers of wood comprising, in combination, a lower work-supportingconveyor means comprising a lower electrode-means, said conveyor meansbeing adapted to move wood in a travel-path. therealong, a frame abovesaid conveyor means, said frame comprising a plurality of spacedelongated support-bars, each of said support-bars having a centralelongated slot therein, a plurality of spaced pivot means ineludingpivots in each of said slots, said pivots being slidable in the slots,said pivot means including means for fixing said pivots in selectedpositions in said slots, a plurality of electrodcbars carried by saidpivots in a lazy-tong ar-- rangement. said lower electrode-means andsaid electrode-bars being relatively insulated, and means comprising aterminal connection for the supply of high-frequency power to saidelectrode-- bars.

6. A dielectric-heating electrode comprising a supporting-bar, aplurality of strips, supportmeans for supporting said strips from saidsupporting bar with the strips in spaced parallel relation to each otherbut oblique to said supporting bar, said strips having faces directed inthe same general direction and adapted to be the boundary of ahighdrequency electric field, said support-means being adjustable tovary the spacing between said strips.

7. A dielectric-heating electrode comprising a plurality of obliquelycriss-crossed strips electrically interconnected, said oriss-crossedstrips being adjustable to vary the angle of oblique crossing of thestrips and to vary the spacing between said strips, and a terminal onsaid dielectric-heating electrode.

8. A dielectric-heating electrode comprising a plurality of layers ofspaced parallel metallic strips, the strips of each layer having facesadapted to bound an electric field and lying substantially in a plane,the strips of a first layer crossing the strips of a second layer, andclamping pivot means at the crossing points of said strips, said stripsbeing electrically interconnected.

9. A dielectric-heating electrode comprising a plurality of layers ofspaced parallel strips, the strips of each layer having laces adapted to10 bound anflelectric" field and-lying substantially ina=plane, thestrips of a-"first:layer crcssing the strips ofasecond layer,-andfastening-means securing the strips together, said "fastening meanscomprising loosenablesupport-members for changing theanglebetweencrossing strips.

1 0. A dielectric'heatingelectrode comprising a framehaving a pluralityof'parallel elongated spaced support-bars, a plurality of elongatedelectrode bars, support=members adjustably securinga firstenrl-of-eachof said electrode-bars to a first of said support-bars. andthe opposite.end to a 4 secondof" said "support-bars; with the" electrode-barsremainingzparallel and'equally spaced with respect to each other inallof the adjusted .positionsthereof, said electrode-bars havingiaces lyingsubstantially in a plane, and being oblique to said support-bars.

11. A. dielectric-heating electrode comprising a frame having a pair ofouter support-bars and an inner support-bar therebetween, saidsupportbars being spaced and parallel on the same general level, aplurality of first electrode-bars extending from and between said outersupportbars, a plurality of second electrode-bars from and between saidinner support-bar and said outer Support-bars, said electrode-bars beingcrossed, said. electrode-bars being in electrically linked relation.

12. A dielectric-heating electrode comprising a frame having a pair ofouter support-bars and an inner support-bar therebetween, saidsupportbars being spaced and parallel on the same general level, saidsupport-bars having slots along their lengths, pivot means adjustablypositionable in said slots, and extending below said support-bars, saidpivot means comprising roller means rollable on the support-bars, aplurality of metallic strips carried by said pivot means below saidrollers with the strips in lazy-tong arrangement, certain of said stripsextending from and between said outer support-bars, and means forpassing electricity to said strips.

13. A dielectric-heating electrode comprising a frame having a pair ofouter support-bars and an inner support-bar therebetween, saidsupportbars being spaced and parallel on the same general level, saidsupport-bars having slots along their lengths, pivot means adjustablypositionable in said slots, and extending below said supportbars, aplurality of metallic strips carried by said pivot means below saidsupport-bars, said pivot means comprising electricity conducting memberselectrically roller connected to said strips, and comprising means forvertically adjusting said strips.

14. An adjustable dielectric-heating electrode comprising a supportingbar having an elongated slot, pivot means adjustably positionable insaid slot, and extending below said supporting bar, said pivot meanscomprising roller means rollable on said supporting bar, a plurality ofmetallic strips carried by said pivot means in lazy-tong arrangementbelow said roller means, said supporting bar comprising ahigh-conductivity memher on which said roller means can roll.

15. An invention including that of claim 12 but further characterized bysaid pivot means comprising a plurality of vertical members carrymg saidstrips, and means for vertically adjustlinng said members with respectto said supporting 16. A dielectric heating-electrode comprising aplurality of spaced elongated obliquely crisscrossed electrode-barsarranged in a plurality of superimposed levels, conducting junctionmeans at the crossing points of said bars, and supporting means movablysupporting said bars for lim ited movement in a general directionmaintaining the associated bars in their common level, the movementraising said levels.

17. An invention including that of claim 15 but characterized further bysaid bars being crisscrossed.

MILTON P. VORE. ROBERT E. KIRBY.

References Cited in the file of this patent UNITED STATES PATENTS NumberNumber Number Name Date Crandell June 30, 1942 Rouy Dec. 15, 1942 Waltonet al. June 11, 1946 Grell et a1 Jan. 28, 1947 Brown Oct. 7, 1947 Grahamet a1 Dec. 28, 1948 Nutt Dec. 20, 1949 Stanton et a1 Feb, 20, 1951 WoodFeb. 27, 1951 FOREIGN PATENTS Country Date Germany June 3, 1931 OTHERREFERENCES Modern Plastics, June 1944, page 118.

